1. Field of the Invention
The present invention relates to a backlight device, and further to a display device and a television receiver including the same.
2. Description of the Related Art
In recent years, a display device that includes a liquid crystal panel as a flat display portion and has many advantages such as thinness and light weight over conventional cathode ray tubes, as typified by a liquid crystal display device, has been becoming the mainstream of a home television receiver, for example. Such a liquid crystal display device includes a backlight device and the liquid crystal panel. The backlight device emits light, and the liquid crystal panel displays desired images by functioning as a shutter with respect to light from a light source provided in the backlight device. The television receiver is configured to display information such as characters and images included in the video signals of a television broadcast on the display surface of the liquid crystal panel.
Furthermore, the above backlight device is classified roughly into a direct type and an edge-light type depending upon the arrangement of the light source with respect to the liquid crystal panel. In the liquid crystal display device including a liquid crystal panel of 20 inches or more, the direct type backlight device is used generally because it can facilitate an increase in brightness and size compared with the edge-light type. More specifically, the direct type backlight device has a configuration in which plural light sources are disposed on a back (non-display surface) side of the liquid crystal panel, and the light sources can be disposed immediately on a reverse side of the liquid crystal panel, which enables a number of light sources to be used. Thus, the direct type backlight device is likely to have high brightness and to be suitable for increasing the brightness and the size. Furthermore, the direct type backlight device has a hollow structure and therefore is light-weight even if it is enlarged. In this regard, the direct type backlight device is suitable for increasing the brightness and the size.
In such a conventional backlight device as described above, a cold cathode fluorescent tube (CCFT) or a light emitting diode (LED) has been used as the light source. The liquid crystal display device provided with color filters of colors of RGB (red (R), green (G), and blue (B)) so as to be color-displayable is required to allow light having all wavelengths in the visible region, namely white light, to be incident upon the liquid crystal panel. Therefore, in the backlight device used in the color displayable liquid crystal display device, a three-band tube or a four-band tube is typically used as a CCFT light source.
Here, the three-band tube is a fluorescent tube having wavelengths of red, green, and blue, while the four-band tube is a fluorescent tube having wavelengths of red, green, blue, and deep red. In the case of the three-band tube, red, green, and blue phosphors are sealed in the tube. In the case of the four-band tube, red, green, blue, and deep red phosphors are sealed in the tube. In either of these cases, at the time of lighting, mixing of light of the respective wavelengths occurs, so that the liquid crystal panel is irradiated with the light (white light) having an emission spectrum in all wavelength regions.
Further, in the case where LEDs are used as the light source, a light-guiding plate, a prism sheet, and the like are used to mix the respective color lights outputted from a red LED, a green LED, and a blue LED (a white LED further may be used) so as to form uniform white light, with which the liquid crystal panel then is irradiated.
However, in the case where light sources having respective wavelength regions of red, green, and blue as described above are used in the conventional backlight device, the following problem arises: color purity decreases due to the interaction with the color filters of colors of RGB provided in the liquid crystal panel.
With reference to FIGS. 14 and 15, the decrease in color purity will be described more specifically.
FIG. 14 is a spectrum diagram showing spectral transmission characteristics of color filters of three colors of RGB. As shown in FIG. 14, the respective spectral transmission spectra of the blue color filter and the green color filter have an overlapped area in the range of about 470 nm to 570 nm. Further, the respective spectral transmission spectra of the green color filter and the red color filter also have an overlapped area in the range of about 575 nm to 625 nm. Because of this, in the case where a light source having emission spectra in all wavelength regions is used, mixing of colors occurs in these overlapped areas, resulting in a problem of decrease in color purity.
For example, FIG. 15A is a spectrum diagram showing an emission spectrum of a three-band tube; FIG. 15B is a spectrum diagram showing a spectral transmission characteristic of a red color filter in the case where this three-band tube is used as a light source; FIG. 15C is a spectrum diagram showing a spectral transmission characteristic of a green color filter in the case where this three-band tube is used as the light source; and FIG. 15D is a spectrum diagram showing a spectral transmission characteristic of a blue color filter in the case where this three-band tube is used as the light source.
As can be seen from FIG. 15C, a spectral transmission curve of the green color filter partially overlaps a wavelength region of blue. This means that a blue component is mixed into light from a pixel that is to be displayed in green. Further, as can be seen from FIG. 15D, a spectral transmission curve of the blue color filter also partially overlaps a wavelength region of green. This means that a green component is mixed into light from a pixel that is to be displayed in blue. Such color mixing phenomenon also occurs in the case where a four-band tube is used as a light source, and the phenomenon has been a cause of deterioration in color purity.
In a conventional backlight device, as described in JP 2003-271100 A for example, a driving method (so-called field sequential driving) has been proposed in which LEDs of three colors of RGB are used as light sources with respect to the liquid crystal display device provided with color filters of three colors of RGB, and the LEDs of the respective colors are caused to blink sequentially so that an image of red alone, an image of green alone, and an image of blue alone are displayed in order in one frame. This conventional example has been thought to be capable of reducing brightness irregularities of respective colors of RGB and improving color purity.
However, the above conventional structure has a problem in that when a frame rate is increased, as in the case where display of high-resolution moving images is performed, the field sequential drive in which a display is performed in such a manner that one frame is divided into three colors becomes difficult. Particularly, in the case of the liquid crystal display device, at least presently, a response speed of liquid crystal is not sufficiently high, making it almost impossible to realize high-quality display of moving images by the field sequential drive.
Therefore, the following has been studied: in place of three types of light sources of RGB, two types of light sources that respectively emit lights of two colors mixable into white color light are used and the display is performed by dividing one frame into two in order that high-quality display of moving images can be achieved with the present response speed of liquid crystal.
However, in the case where the display is performed by using two types of light sources and dividing frames as described above, it is required to switch on respective light sources alternately. Accordingly, this causes a problem that light from light sources of one of the two types tends to be emphasized to a user's eye due to the persistence of vision, thereby being recognized as brightness irregularities. Especially in the case where the liquid crystal panel is fully or partially displayed in white, a problem that either of the above light sources is recognized as an image, i.e., a so-called lamp image, tends to occur, which could cause a significant decline in luminous quality.
As described above, in the conventional backlight device, a problem of the decline in luminous quality arises when both of an improvement in color purity and a configuration corresponding with high-quality display of moving images are pursued at the same time.